Manning's Equation

The Manning's equation finds its roots in the Chezy formula for steady uniform flow:

Where the coefficient C is

This gives the more familiar form:

Where:

• n = Manning's Coefficient
• R = Hydraulic Radius
• S = Slope in ft/ft.

Note:

For metric units use 1.0 in place of 1.49 in the above equation.

There are numerous sources for Values of Manning's n.

Type and description of conduits	N Values
	Min	Design	Max
Channels, Lined
Asphalt concrete, machine placed		0.014
Asphalt, exposed prefabricated		0.015
Concrete	0.012	0.015	0.018
Concrete, rubble	0.016		0.029
Metal, smooth (flumes)	0.011		0.015
Metal, corrugated	0.021	0.024	0.026
Plastic	0.012		0.014
Shotcrete	0.016		0.017
Wood, planed (flumes)	0.009	0.012	0.016
Wood, unplanned (flumes)	0.011	0.013	0.015
Channels, earth
Earth bottom, rubble sides	0.028	0.032	0.035
Drainage ditches, large, no vegetation
<2.5 hydraulic radius		0.04
2.5-4.0 hydraulic radius	0.035		0.04
4.0-5.0 hydraulic radius	0.03		0.035
>5.0 hydraulic radius	0.025		0.03
Small drainage ditches	0.035	0.04	0.04
Stony bed, weeds on bank	0.025	0.035	0.04
Straight and uniform	0.017	0.0225	0.025
Winding, sluggish	0.0225	0.025	0.03
Natural Streams
(a) Clean, straight bank, full stage, no rifts or deep pools	0.025		0.033
(b) Same as (a) but some weeds and stones	0.03		0.04
(c ) Winding, some pools and shoals, clean	0.035		0.05
(d) Same as (c ), lower stages, more ineffective slopes and sections	0.04		0.055
(e) Same as (c ), some weeds and stones	0.033		0.045
(f) Same as (d), stony sections	0.045		0.06
(g)Sluggish river reaches, rather weedy or with very deep pools	0.05		0.08
(h) Very weedy reaches	0.075		0.15
Pipe
Asbestos cement		0.009
Cast iron, coated	0.011	0.013	0.014
Cast Iron, uncoated	0.012		0.015
Clay or concrete drain tile (4-12 in.)	0.01	0.0108	0.02
Concrete	0.01	0.014	0.017
Metal, corrugated	0.021	0.025	0.0255
Steel, riveted and spiral	0.013	0.016	0.017
Vitrified sewer pipe	0.01	0.014	0.017
Wood stave	0.01	0.013
Wrought iron, black	0.012		0.015
Wrought iron, galvanized	0.013	0.016	0.017

Typical Manning's Values, (Design Hydrology and Sedimentology for Small Catchments, Haan, Barfield, Hayes, 1994